Iron aluminum phosphate catalyst and method for dehydrogenating and cracking alkanes and olefins



United States Patent 3,327,012 IRON ALUMINUM PHOSPHATE CATALYST ANDMETHOD FOR DEHYDROGENATING AND CRACKING ALKANES AND OLEFINS Charles R.Noddings, Midland, and Ronald G. Gates, Breckenridge, Mich., assignorsto The Dow Chemical Company, Midland, Mich., a corporation of DelawareFiled Jan. 6, 1964, Ser. No. 335,842 Claims. (Cl. 260-680) Thisinvention concerns a new catalyst and a process employing the catalystfor the dehydrogenation and cracking of aliphatic hydrocarbons,primarily paraflins and olefins, having three or more carbon atoms inthe molecule. It pertains especially to a novel catalyst and a processemploying the catalyst for the dehydrogenation and cracking of parafiinsand olefins having three or more and preferably four carbon atoms in thecarbon chain to form by dehydrogenation and/or carbon bond splitting thecorresponding unsaturated hydrocarbon, e.g., olefins and dienes as wellas the lower carbon chain compounds such as the two and three carbonchain compounds, for example propylene, propane, ethylene, ethane,butene and butadiene when hydrocarbon reactants containing 4 or morecarbon atoms are employed, along with dehydrogenated carbon chaincompounds having the same number of carbon atoms per molecule as thestarting hydrocarbon.

It is, of course, well known that aliphatic hydrocarbons, e.g.,petroleum fractions (mixed hydrocarbons) or individual paraflins orolefins, can be pyrolyzed to obtain a mixture of products comprising asmall, though appreciable, proportion of conjugated diolefins and alarger proportion of shorter chain length unsaturate products. Duringsuch pyrolysis, it is thus evident that several different kinds ofreactions usually occur simultaneously, e.g., dehydrogenation, crackingof the carbon-to-carbon linkages in the molecule to form productscontaining fewer carbon atoms per molecule than the originalhydrocarbon, and polymerization of unsaturated hydrocarbons, so that theproduct is, in most instances a hydrocarbon mixture. An example of suchproduct is cracked-oil gas, containing paraflinic hydrocarbons rangingin chain length from methane to hexane, olefins ranging from ethylene tohexylene, and a small amount, usually less than of less saturatedhydrocarbons such as butadiene, isoprene, piperylene and acetylenichydrocarbons. The difficulties involved in recovering the more usefulproducts from such mixture add greatly to their cost.

It is an object of this invention to provide an improved method for thedehydrogenation and/or cracking of C and higher hydrocarbons, andparticularly C and higher hydrocarbons. Another object of the presentinvention is to provide such a method whereby useful organic products(that is, products other than CO carbon and hydrogen) from the pyrolysis(i.e., cracking and/or dehydrogenation) are obtained in quantities whichincrease the economical value of the products over that of the startingcarbon compounds. A further object is to provide a set of operatingconditions under which the new catalyst may effectively be used for theforegoing purposes. Other objects will be apparent from the followingdescription of the invention.

We have found that an iron-aluminum phosphate containing an average ofbetween 6 and 12 and preferably from about 6 to about 9 atoms of ironper atom of aluminum is, under certain operating conditions, efiectivein catalyzing the thermal dehydrogenation and/or cracking of Chydrocarbons, particularly C and higher hydrocarbons, to C and Chydrocarbons, containing a high proportion of olefins.

The catalyst is prepared by mixing together water-soluble metal salts ofiron and aluminum with a water-soluble form of the ortho-phosphatemoiety (P0 in an aqueous medium under conditions such that the pH iswithin.

the range of from 2 to 7. Material of good catalytic quality is obtainedwhen the iron and aluminum are employed to provide from 6 to 12 moles ofiron per mole of aluminum. Further, while not critical, but desirable,the phosphate moiety is employed in a slight excess over that which istheoretically necessary to combine with the metal ions to form a metalortho-phosphate. It is to be understood that the pH may, but does nothave to be maintained within the operative range during mixing but canbe adjusted, after mixing, by addition of a base or acid as necessary tothe reaction mixture to bring the solution within the desired rangethereby causing precipitation of a catalytic material.

The contacting and mixing of the reactants in accordance with the aboverecitation can be carried out in sev eral manners, such assimultaneously, stepwise or intermittently each either in a batchwise orcontinuous manner.

Examples of salts which may be used as starting materials in preparingthe catalyst are the chlorides, bromides, nitrates, and acetates, etc.,of iron and aluminum. Examples of soluble phosphates that may beemployed as starting materials are disodium phosphate, trisodiumphosphate, dipotassium phosphate, di-ammonium phosphate, etc.

The catalyst can also be prepared in either a batchwise manner or acontinuous manner by feeding separate streams of an alkali, preferablyaqueous ammonia, although other bases can be employed as well as twodifferent bases, and as either a single or as a separate stream anaqueous solution of iron and aluminum salts (in relative proportionscorresponding to between 6 and 12 and preferably about 6 to 9 atoms ofiron per atom of aluminum), and either a separate or as a part of anyone of the aforesaid stream a dissolved ortho-phosphate, into a reactionchamber. The relative rates of flow being adjusted such that theresultant mixture will achieve continuously or upon completion of themixing a pH between 2 and 7. It is desirable to retain within thereaction zone a portion of the iron-aluminum phosphate which forms andprecipitates. This is conveniently achieved by adjusting the outflow ofthe iron-aluminum phosphate precipitated to retain a portion of thefiocculent material which settles rapidly to form, as a lower layer ofthe resultant mixture, an aqueous iron-aluminum phosphate slurry thatcontains 2% by weight or more, usually from 7.5 to 10%, of theiron-aluminum phosphate. The reaction mixture, or preferably the settledlower layer thereof, may be filtered to obtain a filter cake whichcontains 10% or more, usually about 22% of the iron-aluminum phosphate.

In order to obtain a rapid settling iron-aluminum phosphate of goodcatalytic activity, it is important that the two or more streams of theabove-mentioned starting materials flow into admixture with one another,e.g. within a body of the resulting mixture, at relative rates such asto maintain the resultant mixture at a pH value between 2 and 7. Thephosphate precipitated from a mixture of higher pH value which is thenadjusted to about 2 to 7 is of good catalytic activity, but is extremelyslow in settling. The phosphate precipitated from a mixture of pH valuebelow 2 settles rapidly, but is less active as a catalyst for thecracking and dehydrogenation of C hydrocarbons than is phosphateprecipitated from mixtures within the range of 2 to 7 pH values. It alsoappears necessary, in order to obtain an iron-aluminum phosphate productof rapid settling rate, that a portion of the precipitated phosphate beretained in the mixing and reaction zone 30 that, once the process isstarted, the catalyst is being formed and precipitated in the presenceof a slurry of the iron-aluminum phosphate. It is probable that thecatalyst already precipitated serves as nuclei for precipi- PatentedJune 20, 1967' tation of further amounts of catalytic material and aidsin controlling the particle size and physical form of the materialundergoing precipitation, but the invention is not restricted to thistheory as to a reason for the result obtained. Presence of preformedparticles of catalyst during precipitation of further amounts of thelatter is not, of itself, suflicient to cause formation of a rapidsettling product, i.e. it is also necessary that the reaction mixture asa whole be maintained at an average pH value of between 2 and 7.

The procedure in bringing the two streams of starting materials togetherand admixing them also has an influence on the rate of settling of thecatalyst which is precipitated. It is desirable that the points of feed,to the aqueous mixture in the mixing chamber, of the streams of the twostarting material be remote from one another and that the mixture bestirred, or otherwise agitated, during introduction of the startingmaterials. Usually, inlets for the different kinds of starting materialsare separated by a distance of a foot or more. Either starting materialmay, of course, be introduced through a plurality of inlets. It isprobable that these precautions of separating the points of feed of thedifferent starting materials and of agitating the mixture result inactual contact between the starting materials in a zone, or zones, ofapproximaely the pH value which is average for the mixture as a whole,i.e. the procedure just recommended presumably results in formation andcoagulation of iron-aluminum phosphate in zones which are actually at pHvalues between 2 and 7. It will be understood that the minimum distancebetween points of feed of the different starting materials is dependentin part upon the rates of feed, and that it may be less with low ratesof feed than with high rates of feed. An increase in the degree, orefficiency, of stirring of the mixture will also permit a decrease inminimum distance between the points of feed. In actual manufacture ofthe phosphate, the points of feed of starting materials mayadvantageously be separated by a distance of feet or more.

Usually water is employed as the solvent for the starting materials, butother ionizing solvents, e.g. aqueous alcohol, may in some instances beused.

In any event after the reaction is complete the precipitate is separatedfrom the liquor by filtration or decantation and is washed with water,decanting or filtering after each washing. The washing should be carriedout so as to remove as thoroughly as possible readily soluble compoundsfrom the product, since such impurities have a disturbing and erraticaction on the thermal decomposition of hydrocarbons. Of particularattention are the unreacted chlorides or byproduct chlorides which, ifretained in the catalyst, tend to deactivate the latter. The catalystis, at this stage in its preparation, a solid or gellike substance whichis apparently amorphous.

After being washed with water, the product is dried, usually attemperatures between 60 and 150 C. The dried product is hard gel usuallyof yellowish color. The gel may be crushed or otherwise reduced togranules, or smalllumps, and be used directly as a dehydrogenationcatalyst. However, it is preferably pulverized, e.g. to a particle sizecapable of passing a 28 mesh screen, and the powdered product is treatedwith a lubricant and is pressed into the form of pills, tablets, orgranules of size suitable for use as a catalyst, e.g. into the form oftablets of from 1 to /2 inch diameter. The lubricant serves to lubricatethe particles during the operation of pressing them into pills and itsuse permits the formation of pills of greater strength and durabilitythan are otherwise obtained. As the lubricant, we preferably use asubstance capable of being removed by vaporization or oxidation from theproduct, e.g. a substance such as graphite, a vegetable oil, or ahydrocarbon oil, etc.

C and higher hydrocarbons can be cracked and dehydrogenated in thepresence of steam and the catalyst of the present invention attemperatures between 600 and 4 750 C., and in some instances attemperatures as much as 50 C. below or above this range. The reaction isadvantageously carried out at temperatures between 650 and 700 C.

Except for the foregoing limitations, the conditions under which thedehydrogenation reaction is carried out may be varied widely. Also, themethod is operable at atmospheric, subatmospheric, or atsuperatmospheric pressures, provided the hydrocarbon reactants is invaporized form. In some instances, the yield of dehydrogenated productdecreases upon increase of the reaction pressure above atmospheric.However, the ability to operate at an increased pressure is ofconsiderable advantage, since condensation of the reaction products maythereby be facilitated. In general, the proportion of hydrocarbonreacted and also the amount of byproduct formation per pass through thecatalyst bed tend to decrease with increase in the rate of vapor flow,and vice versa.

In producing cracked and dehydrogenated hydrocarbon products'inaccordance with the invention, a reaction chamber is charged with thegranular catalyst and the lubricant employed is removed from thecatalyst. This is usually accomplished by passing an O -containing gassuch as oxygen or air, preferably a mixture of about equal volumes ofair and steam, through the catalyst bed at a high temperature, e.g. 450to 750. When the lubricant used in preparing the catalyst granules is asubstance capable of being vaporized, e.g. a mineral or vegetable oil,the step of treating the catalyst with air may be .preceded by one ofpassing an inert gas or vapor such as steam, nitrogen, or carbon dioxideover the catalyst so as to vaporize at least a portion of the bindingagent from the catalyst granules.

After freeing the catalyst of the lubricant, the catalyst bed is sweptfree of the 0 or air with steam and is heated to the desired reactiontemperature, preferably by passing superheated steam through the same. Amixture of steam and the hydrocarbon reactant, e.g. propene, propane,butylene, amylene, hexylene, butane, pentane, or hexane, having at leastthree carbon atoms, is then passed through the catalyst bed at atemperature between 600 and 750 C., and preferably between 650 and 700C. The usual procedure is to pass the hydrocarbon gas into admixturewith steam which has been superheated to 750 C. or above, i.e. to atemperature suflicient so that the resultant mixture is at the desiredreaction temperature, and to pass the mixture through the bed ofcatalyst. However, the heat may be supplied in other ways, e.g. byforming the steam and hydrocarbon mixture at a lower temperature andpassing the mixture through a preheater to bring it to the desiredtemperature, or by externally heating the catalyst chamber itself. Theyield of olefins is usually highest when from 10 to 20 volumes of steamare employed per volume of the gaseous or vaporized hydrocarbon, but thesteam may be used in smaller or larger proportions if desired. Ashereinafove mentioned, the rate of vapor flow through the catalystchamber may be varied widely, but in practice the flow usuallycorresponds to between 100 and 7 00 liters of the hydrocarbon (expressedas at 0 C. and 760 millimeters pressure) per liter of catalyst bed perhour.

The vapors issuing from the catalyst chamber are ordinarily passedthrough heat exchangers and other cooling devices to condense first thewater and then the hydrocarbon products. By repeatedly recycling theunreacted hydrocarbons, an olefin product may be produced in a 60% yieldor higher and usually in a yield of from 70 to of theoretical or higher.

During use in the process, the catalyst gradually accumulates a smallamount of carbon, or non-volatile organic material, and loses itsactivity. Accordingly, flow of the hydrocarbon starting material isperiodically interrupted and air, admixed with the steam, is blownthrough the catalyst bed, e.g. at temperatures between 450 and 700 C.,and preferably at the dehydrogenating temperamaterial and thusreactivate the catalyst. Usually from 10 ture, to oxidize and remove thecarbonaceous or organic material and thus reactivate the catalyst.Usually from to 30 minutes are required to carry out this reactivationstep. However, if, during compounding of the catalyst into tablet form,an agent having the property of catalyzing the oxidation of carbon isadmixed therewith, the time subsequently required for reactivating thecatalyst with steam and air may be reduced markedly. For instance, theincorporation of one or two percent of chromic oxide in the catalysttablets facilitates reactivation of the catalyst. Other agents havingthe property of catalyzing the burning of carbon are known to the art.

After completing the reactivation step, the catalyst chamber is againswept free of air with steam and the introduction of hydrocarbons,together with the steam, is resumed. Usually, reactivation of a catalystis advisable after from to 60 minutes of use in the dehydrogenationreaction. In practice, two or more catalyst chambers are preferablyemployed in a system provided with connections for passing the reactionmixture alternately through different catalyst beds. One catalyst bed isusually employed in the dehydrogenation reaction while another is beingreactivated. By operating in this manner, the dehydrogenation reactionmay be carried out continuously.

The following examples illustrate the present invention, but are not tobe construed as limiting:

Example 1 In the manner shown in the accompanying drawing, 16.2 grammoles of iron chloride as a 38.5 weight percent aqueous solution thereofwas mixed in a vessel with 2.7 gram moles of aluminum chloride as a 15weight percent aqueous solution and 19.5 gram moles of phosphoric acidas a 75.5 weight percent aqueous solution and the resulting mixture isdiluted with water to a total volume of 95 gallons. Upon completion ofthe addition of the above enumerated chemicals to the vessel reactor, anaqueous 12 weight percent ammonium hydroxide solution was, or had been,added. In some instances, the aqueous ammonium hydroxide was addedtogether with the reactants, in others after addition of all of thereactants, and in still others the phosphoric acid and ammonia werefirst mixed and then admixed with the other reactants. The reaction masswas continuously stirred and base or acid added to produce and maintaina pH of the system between 3.2 and 3.1. In the specific instance, 58.4gram moles of ammonium hydroxide were required to maintain the pH at 3.2at the end of 2.6 hours of reaction. The reaction was consideredcomplete when the final pH remained constant. Thereafter the reactionmass was allowed to settle overnight after which the supernatant liquidabove the precipitate was drawn ofi (approx. 26 gallons decanted) andthe resulting thick slurry filtered and washed with water. The filtratewas discarded. In the specific instance the slurry was washed on aBuchner filter with water until chloride free, then removed and dried at100 C. in a rotary drier. The dry powder was recovered to the extent of105% of the theoretical yield, based on the starting materials used, andwas crushed, mixed with 2% of a lubricant grade graphite and expressedinto pellets about A inch in diameter and inch long. The graphite wasburned ofi by treating the pellets with air and steam at about 650 C.for about 6 hours. The resulting catalyst pellets were tested ascracking and dehydrogenation catalyst at 700 C., 150 v./v. hr. (volumeof gas per unit volume of catalyst per hour) (S.T.P.) with 99% n-butane,3000 v./v. hr. of steam and 1.0 hr. cycle, half of which wasregeneration of the catalyst accomplished by passing v./v. h-r. (S.T.P.)of air and 3000 v./v. hr. (S.T.P.) of steam through the bed. Of thebutane fed to the reactor, there was obtained a yield of 62% of C H(34.5%) and C H (27.5%), plus a 6.5% yield of 1,3-C H and 13.5% yield ofC H all based on the carbon content of the amount of butane converted,which conversion was 19.5%

Example 2 The resulting catalyst pellets from Example 1 were tested asan olefin dehydrogenation catalyst at 650 C., 300 v./v. hr. (S.T.P.)with 99% n-butene, 6000 v./v. hr. of steam and 1.0 hr. cycle, half ofwhich was regeneration of the catalyst accomplished by passing 680 v./v.hr. (S.T.P.) of air and 6000 v./v. hr. (S.T.P.) of steam through thebed. Of the butene fed to the reactor, there was obtained a yield of92.6% l,3-C H based on the carbon content of the amount of buteneconverted, which conversion Was 14.5%.

We claim:

1. The method which comprises dehydrogenating and cracking an aliphatichydrocarbon having at least 3 carbon atoms by passing the hydrocarbontogether with steam at a temperature between 600 and 750 C. in contactwith a catalyst composed of a metal phosphate material consistingessentially of phosphate radicals chemically combined with iron andaluminum in the relative proportions of between 6 and 12 atoms of ironper atom of aluminum which metal phosphate material is preparable bymixing a solution of soluble salts of iron and aluminum with a solutionof a soluble orthophosphate and precipitating said metal phosphatematerial from the mixture at a pH of between about 2 to 7.

2. The method of claim 1 which comprises passing hydrocarbon vaporscontaining a parafiin having 4 carbon atoms and between 10 and 20volumes of steam into contact with the catalyst.

3. The method of claim 1 which comprises passing hydrocarbon vaporscontaining a normal butane into admixture with between 10 and 20 volumesof steam.

4. The method of claim 1 which comprises passing hydrocarbon vaporscontaining a normal butene into admixture with between 10 and 20 volumesof steam.

5. A metal phosphate material prepared by precipitation from a solutionof pH about 2 to about 7 of a watersoluble iron salt, a water-solublealuminum salt, and a phosphate, which metal phosphate material containsbetween about 6 and 12 atoms of iron per atom of aluminum.

References Cited UNITED STATES PATENTS 2,301,913 11/1942 Veltman208--114 2,441,297 5/1948 Stirton 260683.3

2,542,813 2/1951 Heath 252437 3,141,732 7/1964 McCullough et a1. 23105OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic andTheoretical Chemistry, vol. 14, pp. 397 and 410, Longmans, Green andCo., New York, 1935.

DELBERT E. GANTZ, Primary Examiner.

R. H. SHUBERT, Assistant Examiner.

1. THE METHOD WHICH COMPRISES DEHYDROGENATING AND CRACKING AN ALIPHATICHYDROCARBON HAVING AT LEAST 3 CARBON ATOMS BY PASSING THE HYDROCARBONTOGETHER WITH STEAM AT A TEMPERATURE BETWEEN 600* AND 750*C. IN CONTACTWITH A CATALYST COMPOSED OF A METAL PHOSPHATE MATERIAL CONSISTINGESSENTIALLY OF PHOSPHATE RADICALS CHEMICALLY COMBINED WITH IRON ANDALUMINUM IN THE RELATIVE PROPORTIONS OF BETWEEN 6 AND 12 ATOMS OF IRONPER ATOM OF ALUMINUM WHICH METAL PHOSPHATE MATERIAL IS PREPARABLE BYMIXING A SOLUTION OF SOLUBLE SALTS OF IRON AND ALUMINUM WITH A SOLUTIONOF A SOLUBLE ORTHOPHOSPHATE AND PRECIPITATING SAID METAL PHOSPHATEMATERIAL FROM THE MIXTURE AT A PH OF BETWEEN ABOUT 2 TO 7.